Most dc-to-dc converters employing H-Bridge topologies achieve the conversion of a primary dc voltage input to an output regulated dc voltage. Typically, the dc input voltage is converted to an ac voltage (or pulses) by switching devices (transistor, mosfet, insulated gate bipolar transistor or thyristor). The ac voltage is then converted to a regulated dc output voltage. By controlling the duty cycle or the frequency of the ac voltage (or pulses), the desired output voltage or output current regulation can be achieved. These methods of controlling the output are typical of all dc-to-dc converters employing H-Bridge topologies. The introductions of H-Bridge resonant circuits (phase shifted, series, parallel, parallel loaded, etc.) are used for high frequency dc-to-dc conversion. This has been integral to higher power density and efficiency improvements.
H-Bridge converters of present have reduced the losses associated with switching devices (transistor, mosfet, insulated gate bipolar transistor or thyristor) during turn-on and turn-off transitions. These circuits have not shown to correct, in a “passive manner,” the limiting of peak currents in the switching devices. In addition, they do not produce symmetrical currents. These circuits are less than ideal when the converter output (pwm, phase shifted, frequency, and cycle start-stop) is modulated to partial output or near zero output conditions.
However, controlling the current in the converters with an active current protection scheme does not always result in protecting the switching devices from failures. The circuits attempt to limit the rise of current in the switching devices by shutting down the drive circuitry for the interval required. All switching devices have a specified turn off time and current will continue to flow until the turn off time has been reached. The maximum rated current of a switching device can be exceeded during the specified device turn off interval (defined as the storage time and fall time of the device). In this active protection scheme, this interval is the function of the speed in which the control circuit reacts and the device turn off time. These intervals also change with of the junction temperature of the switching devices. This can exacerbate the failure of the active protection scheme.
A number of patents address the advantage of H bridge resonant mode dc-dc converters, e.g. U.S. Pat. No. 4,864,479 (Steigerwald et al) issued Sep. 5, 1989, U.S. Pat. No. 5,442,540 (Hula et al.) issued Aug. 15, 1995, U.S. Pat. No. 5,438,497 (Jain et al.) issued Aug. 1, 1995. The historical problems with these power conversion topologies are that the operation of H-Bridge resonant circuits can be compromised in open or short circuit conditions. When the H-Bridge is operated without symmetrical switch currents and peak current limitations, the peak voltages and currents through the H-Bridge switches can be excessive. This often results in switch failure. H-Bridge resonant circuit improvements have contributed greatly to more efficient and cost effective solutions to dc-to dc converters. The proposed invention addresses the condition specific shortcomings described above.